1. Field of the Invention
The invention relates to a design for and methods for the manufacturing of a stress concentrating structure for applications in MicroElectroMechanical System (MEMS) sensors in particular with respect to microphone and pressure sensor involving the entire or partial composition of all electronic and mechanical elements of the device functions on a silicon substrate or any other material.
2. Description of the Background Art
Smaller size, better performance and lower cost per function are the major driving forces behind the modern electronic industries. To achieve such goals, the industry is attempting to integrate more functions, including mechanical sensors and their sensing electronics, into miniaturized chips.
The most common method for transduction in MEMS microphones and pressure sensors are based upon the capacitance sensing method. This method relies on the measured quantity inducing a proportional displacement in one electrode of a capacitance structure. The capacitance change can then be converted into an equivalent electrical signal.
The disadvantages and difficulties of the conventional art for the realization of stand alone sensor or single chip sensor for microphones, pressure sensors or inertial sensors (but not limited to the above) based on capacitance sensing structures are explained and illustrated by the capacitive MEMS microphone in FIGS. 1 and 2.
FIGS. 1 and 2 illustrate a conventional art construction of a capacitive MEMS microphone 10. It includes a thin bottom plate electrode 12 or membrane formed on a pedestal 13, usually made of silicon. A top plate electrode 11, usually larger in lateral dimension, thicker and perforated, is supported on top of the bottom plate 12 by means of pillars 14. A hollow cylindrical hole 15 of the same dimension as the bottom plate 12 is etched through in the pedestal 13. When a sound wave 16 impinges on the bottom plate 12 either through the hole 15 or from the top through the perforated top electrode 11, the pressure (P˜1 mPa to 10 Pa) causes the bottom plate 12 to deform and changes the distance between the top and bottom electrodes 11, 12. This causes the capacitance between the electrodes 11, 12 to change. A circuit picks up this capacitive change and transduces into an electrical signal.
FIG. 2 illustrates a cross sectional view of the microphone of FIG. 1. The bottom electrode 12 is flexible and changes its distance to the top electrode 11 when a sound wave 16 impinges. The capacitance is a inverse function of inter-electrode distance d. The initial value of d, which determines the static capacitance of the capacitor
      (          C      =                                    ɛ            0                    ⁢          A                d              )    ,can change substantially when there are internal stresses in the top and bottom plates. The change of d with pressure, which shows up as sensitivity of the microphone, is a complex function that depends on the dimension and shape of the bottom electrode 12, its thickness, and the material constant of the electrode 12. These parameters are often not very well controlled functions of a manufacturing technology.
Silicon MEMS microphones are increasingly replacing the Electret Condenser Microphones (ECM) in electronic products because of its smaller size and ruggedness in SMT assembly. Almost all commercial realization of the MEMS microphone uses a two chip approach. It includes a silicon MEMS based capacitive membrane chip connected to a pre-amplifier chip. The two chips are assembled onto a suitable substrate such as a FR-4 PCB and further protected by a packaging cover.
The typical electrical parameters of a silicon MEMS microphone are:
Sensitivity−42dBV/PascalSignal/Noise Ratio60dBHV Generator10-12V +/− 0.25 VAmplifier Output Noise (@ O dB gain)<10μVCurrent Consumption150μASupply Voltage Range1.5V-4.5 V
The MEMS microphone membrane works on the principle of a pressure sensitive capacitor, in which one plate (the flexible plate) is made to bend under the pressure of a sound wave 16. This alters the capacitance of the capacitor, which can be sensed when a voltage is imposed across. This function is illustrated in FIG. 1.
The typical physical parameters of a capacitive MEMS microphone membrane are:
Inter-electrode 1.5-2μmdistance dFixed electrode thickness5-20μmFlexible electrode thickness0.4-0.7μmX-Y dimension<800μmDie dimension<1.2mm × 1.2 mm
For the single chip integration, almost all the attempts have been based upon using the capacitive MEMS membrane as the sensor, despite the many difficulties and drawbacks of implementing the capacitive MEMS membrane with the integrated circuit chip due to reasons described below. For these reasons the cost of the single chip MEMS microphone remains high and is unable to compete with the two chip MEMS microphone or the ECM microphone.
In the MEMS microphone capacitive membrane, the capacitor has two plates separated from each other usually by air plus an insulator on the plates and supported on insulating material. In order to obtain a pre-determined capacitance and hence the important sensor parameter of sound pressure to voltage sensitivity of the microphone, the distance between the two plates must be made to an exact specification. This has two implications: the fixed and flexible electrodes must experience low stress so that they would not bend and alter the inter-electrode distance between them; and the inter-electrode gap of air, determined primarily by the insulating stud between the two plates, and the thickness of dielectric layers of silicon oxide/nitride must be made constant and repeatable. The two imperfections must be controlled and contribute to the complexity of the design and affects the manufacturing yield of a capacitive MEMS microphone.
Internal stress develops in all polycrystalline films formed by chemical or physical deposition methods. The notable materials are usually polysilicon, silicon dioxide and silicon nitride. The stress can be tensile or compressive. Its formation and magnitude depends on the thickness, method of deposition and deposition parameters. The magnitude of the stress is very difficult to control and the variation can be large, up to + and −100%. Silicon nitride has a larger stress compared to polysilicon and silicon oxide. Silicon rich nitride has a lower stress but because its oxidation leads to silicon oxynitride which is difficult to etch, its use within the IC production process is not mainstream and often avoided. Since oxide and nitride are not conductors, polysilicon is often used as material for the conducting plates in conjunction with nitride or oxide because polysilicon must be protected from the environment due to its richness in grain boundaries, which can lead to point of weakness when the material is exposed to etchants. The stress from such composites is even harder to control. Thus in the early development of the capacitive MEMS microphone sensors, much attention has been given to constructions and designs which lead to stress relief or stress tolerant structures.
The thickness of thermally grown films can be controlled with great accuracy. Such growth, however, takes place at an elevated temperature (usually >900° C.) and as to be seen later, may not be a desirable part of the capacitive sensor formation process unless the sensor plates are formed as part of the IC process itself. If true, this will compromise and introduce variations to the basic IC process which adds significant cost and complexity to the already complicated IC process. Thus inter-electrode control tends to be done by a low temperature plasma enhanced chemical vapor deposition process at lower temperature, sputtering, physical evaporation or even spin on of polymers or inorganic substances such as a spin on oxide. The thickness control using these methods ranges from fair to poor.
In the manufacturing of MEMS capacitive sensing structures, the surface tension of water or a cleaning fluid plays a special role. Stiction, referring to the sticking together of two opposite hydrophilic surfaces through surface tension force from water, occurs during the drying process subsequent to wet etching and cleaning. The surface tension in a small droplet of water would pull the two surfaces together during drying and the surface atomic force can keep the surface attached, therefore destroying the function of the sensor. Since aqueous processing is an essential part of the MEMS manufacturing as well as a cutting process to eliminate silicon particle contamination, stiction places serious constraints on the realization of capacitive sensing structures and post-process fabrication steps. Stiction is avoided by using HF vapor etching of sacrificial oxide spacer or super-critical drying in aqueous cleaning. But use of the latter is not possible during wafer cutting, adding a further complication to the manufacturing process.
In consideration of a MEMS silicon capacitive sensing structure for single chip integration, a particular sequence of high temperature and low temperature material processing, the compatibility of the different materials with each other with respect to chemical and physical processing, and the number of process steps introduced on top of the basic integrated circuit process to add the mechanical element. In general, the mechanical element is added as the last steps, as a so called post-CMOS process, to the IC process. To avoid melting the metallic aluminum interconnect, the processing temperature must be kept below 450° C. This places constraints on the choice of materials that can be used for the mechanical structures, conducting or non-conducting, their properties and the methods of deposition.
In a single chip MEMS microphone the rest of an integrated circuit (IC) is sealed under PECVD nitride and oxide. An opening is made to the membrane area. For realizing capacitive membrane, there are about five material layers with etching and five lithography steps involved in the formation of this two plate capacitor membrane. Even if some of these layers and steps can be shared with the basic IC process, the process is still a relatively complicated endeavor. Controlling the thickness of the lower electrode is critical. It determines the bending of the circular/rectangular plate which determines the sensitivity. Also, controlling the spacing of the inter-electrode spacing is critical. The material must be compatible with the release process. Oxide and polysilicon grain boundary is attacked or weakened by HF vapor, a dry release chemical. Thus an inter-electrode sacrificial spacer material must be selected carefully to accommodate deposition temperature, thickness control and etching compatibility with surrounding materials. Not many combinations of materials are available as solutions. They are often very complex.
The circuitry of a capacitive MEMS microphone includes a low noise accurate charge pump, a low noise amplifier with insulated gate input and an accurately controlled impedance for biasing the amplifier and controlling the noise of the MEMS sensor-amplifier combination. The circuit is small by VLSI standards but its size is not linearly scalable with respect to technology line-width shrinkage. In general, the noise of the capacitive MEMS microphone is limited by kT/C noise where C is the capacitance of the MEMS sensor and parasitic capacitances. For C=1 pF, the noise voltage (from 100 Hz to 10 kHz) is above 10 microvolts. It increases with decreasing C, 1.4 times for halving the capacitor value. There is thus a fundamental size of the MEMS capacitor membrane beyond which the sensor cannot be shrunken for deterioration of the signal to noise ratio of the sensor. A common fallacy in designing capacitive microphone is the assumption that with decreasing C, the sensitivity can be restored by making the flexible plate thinner. But such an act would decrease the maximum voltage at which the sensor can operate due to the decreasing pull-in voltage of the electrostatic attraction, thus negating any improvement that comes from making the capacitor thinner. The SNR of the MEMS capacitive microphone is often given in dB and it is the ratio of the voltage signal output of the microphone at 1 Pascal sound pressure and the noise output of the microphone without input A-weighted. The A-weighting eliminates a large portion of the amplifier noise at low frequency. Prevalent of this SNR is around 60 dB. For capacitive MEMS microphone, A-weighting improves the SNR by about 2-4 dB. This figure also depends on the low frequency characteristics of the amplifier.
An alternative method of transduction is based upon piezoresistive sensing in a single silicon plate. The method has been applied to plates of MEMS pressure sensors where piezoresistors are embedded at the edges of a square or circular silicon plate. Applied pressure causes stress at the edge depending on the length/thickness of the plate and this stress can be transduced by the piezoresistors.
The disadvantages of the plate/piezoresistive structure are that it is extremely hard to control the uniformity and thickness of a large plate during the etching process. A non-uniformity of the plate causes errors in the reading of the piezoresistor. The sensitivity of the pressure sensor is determined by the thickness of the plate. If this thickness is limited to a maximum, then the sensitivity of detection drops. For this reason, the structure is not applied in any commercial realization of a MEMS microphone or ultra-sensitive pressure sensor. It further limits the size of a pressure sensor in that a minimal force has to be created through the area of the plate to create a detectable stress. For these reasons, the plate/piezoresistive structure is less used in favor of the capacitance sensing structure.